Cryostats utilizing the well-known Joule-Thomson effect or cooling cycle are shown in U.S. Pat. Nos. 3,006,157, 3,021,683, 3,048,021, 3,320,755, 3,714,796, 3,728,868, 4,237,699 and 4,653,284. All of the cryostats shown in the enumerated patents rely upon a heat exchanger wherein high pressure fluid is conducted along a path which is in heat exchange with the cooled lower pressure gas returning after expansion through a Joule-Thomson orifice. In all of the prior art devices, the heat exchanger is constructed by wrapping a finned tube around the outside of a mandrel, the finned tube terminating in a Joule-Thomson orifice. The wrapped tube heat exchanger is disposed in a dewar or other sleeve so that the high-pressure gas conducted down through the finned tube exiting the Joule-Thomson orifice which has expanded to produce refrigeration is conducted countercurrently over the outside of the finned tube to precool the in-coming high pressure gas. One of the problems with heat exchangers of this type which are embodied in cryostats is the lack of fast cool down (response) time. This is especially a problem with cryostats used by the military to cool infra-red detectors in guided missiles. As is well-known, guidance begins when the missile leaves the launcher and that the missile must be fired as soon as possible should the need arise. In general, cryostats of the type employing the finned tube heat exchanger must be operational several seconds before the missile is launched so that it can provide the necessary refrigeration to cool the IR detector and thus, have the missile guidance system in condition to guide the missile to the target. The best response time with a conventional finned tube heat exchanger has been to reach a temperature of 92.4.degree. Kelvin (.degree.K.) in 2.5 seconds at the Joule-Thomson orifice.
A heat exchanger using stacked screens was proposed by G. Bon Mardion and G. Claudet in an article appearing in CRYOGENICS, September 1979 entitled "A Counterflow Gas-Liquid Helium Heat Exchanger with Copper Grid". The authors do not disclose how such a heat exchanger would be constructed for use in a fast cool-down cryostat. Mardion and Claudet were not concerned with the mass of the heat exchanger because of the wire sizes employed, thus a fast response (cooldown) time would not be observed for this heat exchanger.